1. Field of the Invention
The invention relates generally to wireless communications and more specifically to a system and method employing standard DECT hardware to operate in the Industrial-Scientific-Medical (ISM) band.
2. Description of Related Art
The following background information is provided to aid in the understanding of the application of the present invention and is not meant to be limiting to the specific examples set forth herein. The so-called “Industrial-Scientific-Medical (ISM)” band allows for unlicensed wireless operation in the 2.4 GHz spectrum (as well as the 900 MHz and 5.8 GHz spectrum) provided however, that the power output is less than one watt and that some form of spread spectrum technology, i.e. Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS) or a hybrid of FHSS and DSSS, is used to minimize interference. In the United States, 47 CFR Part 15 specifies the use of at least seventy-five (75) hopping frequencies between 2.4 GHz and 2.4835 GHz and a minimum hop rate of 2.5 hops per second for FHSS systems. Several schemes have been proposed to use the ISM spectrum for wireless data communication applications, such as for Wireless Local Area Networks (WLANs).
In its 802.11 standard, the IEEE promulgated, inter alia, FHSS and DSSS definitions for the physical layer of a WLAN. For FHSS in North America and most of Europe, IEEE 802.11 requires 79 channels in 1 MHz steps beginning at 2.402 GHz and ending at 2.480 GHz with a minimum frequency hop of 6 MHz. FIG. 9 depicts the IEEE 802.11 protocol for packetizing information in a FHSS WLAN. One-hundred-twenty-eight (128) bits (a 96 bit preamble and 32 bit header) are sent to assist in synchronizing after a carrier hops from one frequency to the next. Payload data then follows in sizes ranging from 1 to 4095 bytes.
An example of an ISM FHSS WLAN is the HomeCast™ Open Protocol (HOP™) from Alation Systems Inc. of Mountain View, Calif. embodied in one form as the HomeFree™ Wireless Network product from Diamond Multimedia Systems, Inc. of Vancouver, Wash. The HOP system provides 79 channels and a maximum data throughput of 1 Mbps but employs a proprietary ISM baseband processor, requires a host processor to implement a software MAC, and does not support voice communications.
The so-called “PRISM I” chipset from the Intersil Corporation of Melbourne, Fla., is an example of a DSSS WLAN implementation in compliance with the IEEE 802.11 standard. The PRISM I chipset comprises six discrete integrated circuits, requires a proprietary baseband processor, and while maintaining IEEE 802.11 compliant, can only achieve a maximum data throughput of 2 Mbps and does not support voice communications. It can be seen therefore, that the PRISM I DSSS solution while improving data throughput, also increases chip count and cost and locks the design into proprietary hardware.
By way of further background, reference is made to FIG. 1 that depicts the prior art Digital Enhanced Cordless Telecommunications (DECT) standard protocol promulgated by the European Telecommunications Standards Institute (ESTI). The DECT standard defines a Multiple Carrier, Time-Division-Multiple-Access (TDMA), Time-Division-Duplex (TDD) protocol with ten channels (carrier frequencies) between 1881.792 MHz and 1897.344 MHz spaced 1.728 MHz apart. Each of the ten channels supports a ten-millisecond frame comprised of twenty-four time slots. TDD is provided by allocating twelve of the twenty-four slots for base station to cordless handset communications and the other twelve slots for cordless handset to base station communications. Each time slot comprises 480 bits with a 32-bit preamble for synchronization, 388 bits for data and 60 bits for guard time. The 388 data bits are further divided into an A-field, a B-field and 4 parity bits for error detection. The A-field comprises an 8-bit header, 40 bits of control information and 16 cyclic redundancy check (CRC) bits while the B-field provides 320 bits of data.
For speech applications, analog signals are digitized and encoded using adaptive differential pulse code modulation (ADPCM). Frequency hopping is employed to avoid interference by periodically assigning a different one of the ten channel frequencies to each of the twenty-four time slots. A form of frequency shift keying known as Gaussian filtered, minimum shift keying (GMSK) is used to modulate the transmitted signal to provide continuous phase transitions between two adjacent symbols.
DECT enabled products are ubiquitous in Europe ranging from telephones and WLANs to cordless terminal mobility (CTM) applications wherein a cordless handset operates with both private and public base stations. Unfortunately in the United States as well in other countries, the DECT spectrum between 1881.792 MHz and 1897.344 MHz is licensed for Personal Access Communication Systems (PACS) and is not available for unlicensed applications such as WLANS.
From the foregoing it can be seen that there is a need for a system and method employing standard DECT hardware but operates in the unlicensed Industrial-Scientific-Medical (ISM) spectrum.